In the book The 21 Balloons, how many people did Krakatoa feed each night?

The short answer is "eighty." Here are the details:

In The Twenty-One Balloons by William Pène du Bois, twenty families live on the island of Krakatoa, and each family has "one boy and one girl," in addition to a mother and father. Each family is renamed after a letter of the alphabet, so that the members of the first family, for example, are called Mrs. A. and Mr. A, with the kids called A-1 (the son) and A-2 (the daughter).

This means that four people make up each of the twenty families, so in total, eighty people live on the island. They all dine every night in a restaurant (with each family running an individual restaurant), so that means that all eighty of them are eating each night on the island.

In the story, Mr. F. elaborates on how the restaurants are set up: "'There are twenty restaurants around the village square hen. We lettered them, A, B, C, D, E, and F, all around the square up to T, the twentieth house.'"